Principled BSDF subsurface scattering values for skin material
Is the Subsurface parameter value always at 1.0 for a skin material? If not, does it need to be mapped with a texture?
  


Some parts of the skin have more SSS than others, therefore a texture helps... and rarely a skin gets 100% SSS. So a value of 1 is unrealistic.


  
  Would the Subsurface Radius parameter be constant across the face of a real person? If not, how can I map this value with a texture given it accepts a Vector input?
  


Also another non-constant value. Depending on the density and depth of the skin, if there's bone or flesh under it, etc; this value can change. Here you can play in conjunction with the SSS color, as this parameter is only there to help SSS color to get higher radius. Note that Cycles simply multiplies both Radius and Color. If your SSS has a maximal radius less than 1, you can just use the SSS color. Vector texturing is possible in formats like EXR or HDR (and since in this case, the radius is allways positive, you can use also PNGs or any +16bpp format)


  
  Is the Subsurface Color intended to have a subdermal map as its input?
  


Same thing as the above.


  
  Is there a document that lists parts of the skin and common SSS values for them?
  


There are plenty of artists who shared their common values... But this is not rocket science, and there's always plenty of room for artistic interpretation.